Was it simply used as a piece of backdrop in the Dark Tower series?
I remember decades ago reading a lot of SK’s novels and there were little hints and cameos that seemed to hint at a bigger plot, the stories would bleed into each other.
And I seem to recall some interview where SK said it meant something and it would be revealed later.
Was it just the multiverse thing in Dark Tower?
Have you read The Dark Tower series? Honest question - the answer to your OP is “yes, it’s because of The Dark Tower”, but I wasn’t sure if you’d read the series, but just wasn’t impressed with the connectivity with the rest of King’s canon.
It just adds texture, evoking a larger world than that which exists in one single book. If you get the connection, great. If not, you usually haven’t missed much–but the allusion often will compliment the loyal reader who picks up on a throwaway detail, place name, or character. It’s fun.
Faulkner did it with a larger quantity of his writing, much of which took place in the Yoknapatawpha county in Mississippi, where legacies and events overlapped and entwined in interesting ways.
There were characters and events that appeared or were referenced in seemingly unrelated stories/novels. Nothing major, usually just a line or two. It’s been ages since I’ve read anything of King’s, so I’m hard pressed for examples other than Randall Flagg, who has appeared in The Eyes of the Dragon, The Stand, and The Dark Tower series. And of course Castle Rock, Maine has been the setting for most of his early work. The Dark Tower and (I think) The Talisman gave some deeper glimpses into the connections.
But I don’t think it’s necessarily any big “link”, I think it’s just King establishing that his stories all take place in the same dark, creepy universe.
In Insomnia, there’s an allusion to the big flood in Derry, which happened in It.
Thomas from Eyes of the Dragon is referred to in The Dark Tower series.
Black House (sequel to The Talisman) goes into detail about what the Breakers do, and the end makes it seem like Jack Sawyer has “flipped” into the world inhabited by Roland and company.
The Tommyknockers refers to King himself. Near the beginning, the narrative says that people like Bobbi Anderson’s books better than that other Maine writer who writes those horror books.
This. Tolkien used this same thing very successfully in his writings.
In 11/22/63 the main characters visits Derry and actually meets a couple of the kids (I don’t remember who, I read that book before It).
Largely none–the connections are neat, but he totally fucked up the payoff he was building towards. For example, the ending of Insomnia dribbles off with the understanding that all will be explained in the Dark Tower series, but in the last or second-to-last book of the Dark Tower series, he handwaves it away by saying something like “It was a false connection. So…jes’ foolin’!” Total cheat.
Ditto in IT. There’s all that “Voice of the Turtle” stuff and you later find out that the Turtle is one of the guardians of the Tower and…nothing.
You can appreciate (or not. I do.) the connections on their own merit, but if you’re looking for a grand payoff*, you’re going to be badly disappointed.
*I was expecting to see the “how and why” these stories/worlds were connected through the Tower. The utterly dismissive handwave of “Just foolin’” was a huge letdown for me.
Tommyknockers also connects to IT because one of the “possessed” people have to drive to Derry to get something (batteries? I don’t remember any more and hated Tommyknockers and as he’s leaving, he comments how damned creepy that town is.
IIRC, a Takuro Spirit and possibly Nozz-a-la cola showed up in 11/22/63. Characters in one of the latter Dark Tower books actually give Roland and the ka-tet a copy of Insomnia and suggest it is Really Important (they lie–its primary use is as a cure for the condition described by its title). It is, as everyone says, a fun little Easter Egg, especially for us Dark Tower fans.